How do you replace repeating elements in an array? - arrays

I have an array:
[1, 4, 4, 4, 2, 9, 0, 4, 3, 3, 3, 3, 4]
and want to replace the repeating values with a string "repeat". The repeated 4 at indices 1, 2, 3 and 3 at indices 8, 9, 10, 11 should be replaced. I should get:
[1, "repeat", 2, 9, 0, 4, "repeat", 4]
How is this accomplished?

Here are two ways you could do that.
#1 Use Enumerable#chunk:
arr = [1,4,4,4,2,9,0,4,3,3,3,3,4]
arr.chunk(&:itself).map { |f,a| a.size==1 ? f : "repeat" }
#=> [1, "repeat", 2, 9, 0, 4, "repeat", 4]
The steps:
enum = arr.chunk(&:itself)
#=> #<Enumerator: #<Enumerator::Generator:0x007febc99fb160>:each>
We can view the elements of this enumerator by converting it to an array:
enum.to_a
#=> [[1, [1]], [4, [4, 4, 4]], [2, [2]], [9, [9]], [0, [0]],
# [4, [4]], [3, [3, 3, 3, 3]], [4, [4]]]
Object#itself was added in Ruby v2.2. For earlier version you would use
enum = arr.chunk { |e| e }
It is now a simple matter to map the elements of enum as required:
enum.map { |f,a| a.size==1 ? f : "repeat" }
#=> [1, "repeat", 2, 9, 0, 4, "repeat", 4]
#2 Use Enumerable#slice_when
arr.slice_when { |e,f| e !=f }.map { |a| a.size==1 ? a.first : "repeat" }
The steps:
enum = arr.slice_when { |e,f| e !=f }
#=> #<Enumerator: #<Enumerator::Generator:0x007febc99b8cc0>:each>
a = enum.to_a
#=> [[1], [4, 4, 4], [2], [9], [0], [4], [3, 3, 3, 3], [4]]
a.map { |a| a.size==1 ? a.first : "repeat" }
#=> [1, "repeat", 2, 9, 0, 4, "repeat", 4]
slice_when was introduced in Ruby v.2.2.

Related

Split array in groups of n, m, o groups or more

Is there any method to split an array like this?
[1, 2, 3, 4, 5, 6, 7, 8, 9].split(3, 4, 2)
#=> [[1, 2, 3],[4, 5, 6, 7],[8, 9]]
Immutable version with λ:
▶ splitter = ->(array, *parts) do
parts.reduce([[], 0]) do |acc, i|
right = acc.last + i
[acc.first << (acc.last...right), right]
end.first.map { |r| array[r] }
end
#⇒ #<Proc:0x0055ae3d9ae7c8#(pry):18 (lambda)>
▶ splitter.((1..9).to_a, 3, 4, 2)
#⇒ [[1, 2, 3], [4, 5, 6, 7], [8, 9]]
No, there is none, but you can easily write one yourself.
class Array
def in_groups_of_n(*sizes)
sizes.map(&method(:shift))
end
end
Example:
arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
arr.in_groups_of_n(3, 4, 2)
# => [[1, 2, 3], [4, 5, 6, 7], [8, 9]]
Demonstration
In case you want a none-destructive version, you can use a dup method:
class Array
def in_groups_of_n(*sizes)
duplicate = dup
sizes.map { |size| duplicate.shift(size) }
end
end
arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
arr.in_groups_of_n(3,4,2)
# => [[1, 2, 3], [4, 5, 6, 7], [8, 9]
arr
# => [1, 2, 3, 4, 5, 6, 7, 8, 9]
Demonstration
Here's a naive Array implementation:
class Array
def multi_split(*sizes)
r = []
e = self.each
sizes.each do |size|
t = []
size.times do
t << e.next
end
r << t
end
r
end
end
p [1, 2, 3, 4, 5, 6, 7, 8, 9].multi_split(3, 4, 2)
# [[1, 2, 3], [4, 5, 6, 7], [8, 9]]
#Stefan mentioned it might make sense to implement it on Enumerable:
module Enumerable
def multi_split(*sizes)
Enumerator.new do |yielder|
e = self.each
sizes.each do |size|
yielder << Array.new(size){ e.next }
end
end
end
end
p [1, 2, 3, 4, 5, 6, 7, 8, 9].multi_split(3, 4, 2).to_a
# [[1, 2, 3], [4, 5, 6, 7], [8, 9]]
Another option (lossless in event the splits are not equal to the array size
def split_at(arr,splits)
rest = arr.last(arr.size - splits.reduce(:+))
enum = arr.to_enum
splits.map do |n|
n.times.map { enum.next }
end.concat(rest.empty? ? [] : [rest])
end
Then called as
split_at (1..9), [3,4,2]
#=> [[1, 2, 3], [4, 5, 6, 7], [8, 9]]
split_at (1..22), [3,4,2]
#=> [[1, 2, 3], [4, 5, 6, 7], [8, 9], [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]]
Example
class Array
def split_by_number(*sizes)
sizes.each_with_object([]) { |n,a| a << [a.empty? ? 0 : a.last.sum, n] }.
map { |start, nbr| self[start, nbr] }
end
end
[1, 2, 3, 4, 5, 6, 7, 8, 9].split_by_number 3, 4, 2
#=> [[1, 2, 3], [4, 5, 6, 7], [8, 9]]
Note that
[3, 4, 2].each_with_object([]) { |n,a| a << [a.empty? ? 0 : a.last.sum, n] }
#=> [[0, 3], [3, 4], [7, 2]]

Ruby - collect same numbers from array into array of arrays

I have created a very ugly script to collect same numbers from an array. I don't think this is a very Ruby way :) Anyone could provide a more clean solution?
ar = [5, 5, 2, 2, 2, 6, 6]
collections = []
collect_same = []
while ar.length > 0
first = ar.values_at(0).join.to_i
second = ar.values_at(1).join.to_i
if ar.length == 1
collect_same << ar[0]
collections << collect_same
break
else
sum = ar.values_at(0, 1).inject {|a,b| a + b}
if second == first
p collect_same << ar[0]
ar.shift
else
collect_same << ar[0]
collections << collect_same
collect_same = []
ar.shift
end
end
end
p collections
The output:
=> [[5, 5], [2, 2, 2], [6, 6]]
Note, that in primary array same numbers always goes one after another.
So I wouldn't have primary array like this - ar = [1, 2, 1, 2]
Using chunk_while:
[5, 5, 2, 2, 2, 6, 6].chunk_while(&:==).to_a
#=> [[5, 5], [2, 2, 2], [6, 6]]
Ruby prior to 2.3:
[5, 5, 2, 2, 2, 6, 6].each_with_object([]) do |e, acc|
acc.last && acc.last.last == e ? acc.last << e : acc << [e]
end
#=> [[5, 5], [2, 2, 2], [6, 6]]
In case if you want to do it without order:
ar.group_by(&:itself).values
=> [[5, 5], [2, 2, 2], [6, 6]]
[5, 5, 2, 2, 2, 6, 6].slice_when(&:!=).to_a
#=> [[5, 5], [2, 2, 2], [6, 6]]
One could perhaps say that Enumerable#chunk_while and Enumerable#slice_when are ying and yang.
Prior to Ruby v2.3, one might write
[5, 5, 2, 2, 2, 6, 6].chunk(&:itself).map(&:last)
and prior to v2.2,
[5, 5, 2, 2, 2, 6, 6].chunk { |n| n }.map(&:last)
just another oneliner
arr = [5, 5, 2, 2, 2, 6, 6]
arr.uniq.map {|e| [e]*arr.count(e) }
# => [[5, 5], [2, 2, 2], [6, 6]]

Finding index and size of consecutive repeated elements in an array

An array consists of 1, 2, and 0s. I am trying to identify the maximum repetition and its starting index within the array.
Example:
2 2 1 0 2 2 2 0 1 1
The method should accept an integer arguement, which can be one of the numbers 1 or 2
If we demonstrate these inputs on above array, the outputs would be:
find_duplicates(2)
=> 3,4
find_duplicates(1)
=> 2,8
where the first number indicates the size of the duplication, and second is the starting index of it.
I tried looping through the array and compare with arr[i+1] or arr[-1], but this is not the correct approach. Any help will be greatly appreciated.
Edit:
I had not pasted what I had tried at the time I asked the question, this is not something I would do if I could feel some confidence on the way I followed:
def find_status(arr,participant)
status = Array.new
#arr is a two dimensional array
for i in 0...arr.length do
current_line=arr[i]
cons=0
for j in 0...current_line.length do
#I worked on lots of if/else/case statements here, this is just one of them
if current_line[j] == participant
cons+=1 #count consecutive
if current_line[j]!=participant
cons=0
end
end
status[i] = cons
end
end
return status
end
def max_run(arr, target)
_,b = arr.each_with_index.
chunk { |n,_| n==target }.
select { |tf,_| tf==true }.
max_by { |_,a| a.size }
b ? [b.size, b.first.last] : nil
end
arr = [1,1,2,2,2,3,1,1,1,1,2,2,2,2,3,3]
max_run(arr,1) #=> [4, 6]
max_run(arr,2) #=> [4, 10]
max_run(arr,3) #=> [2, 14]
max_run(arr,4) #=> nil
For target = 2, the steps are as follows:
enum0 = arr.each_with_index
#=> #<Enumerator: [1, 1, 2, 2, 2, 3, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3]
# :each_with_index>
We can see the elements that will be generated by this enumerator by converting it to an array:
enum0.to_a
#=> [[1, 0], [1, 1], [2, 2], [2, 3], [2, 4], [3, 5], [1, 6], [1, 7], [1, 8],
# [1, 9], [2, 10], [2, 11], [2, 12], [2, 13], [3, 14], [3, 15]]
Continuing,
enum1 = enum0.chunk { |n,_| n==target }
#=> #<Enumerator: #<Enumerator::Generator:0x007f9beb9b0850>:each>
Carefully examine the return value here. You can think of enum1 as a "compound enumerator". It will generate the following values:
enum1.to_a
#=> [[false, [[1, 0], [1, 1]]], [true, [[2, 2], [2, 3], [2, 4]]],
# [false, [[3, 5], [1, 6], [1, 7], [1, 8], [1, 9]]],
# [true, [[2, 10], [2, 11], [2, 12], [2, 13]]], [false, [[3, 14], [3, 15]]]]
Continuing,
c = enum1.select { |tf,_| tf==true }
#=> [[true, [[2, 2], [2, 3], [2, 4]]],
# [true, [[2, 10], [2, 11], [2, 12], [2, 13]]]]
_,b = c.max_by { |_,a| a.size }
#=> [true, [[2, 10], [2, 11], [2, 12], [2, 13]]]
b #=> [[2, 10], [2, 11], [2, 12], [2, 13]]
b ? [b.size, b.first.last] : nil
#=> [[2, 10], [2, 11], [2, 12], [2, 13]] ? [4, [2,10].last]
#=> [4, 10]
a = [2, 2, 1, 0, 2, 2, 2, 0, 1, 1]
longest_sequence =
a.each_index.select{|i| a[i] == 2}.chunk_while{|i, j| i.next == j}.max_by(&:length)
# => [4, 5, 6]
[longest_sequence.length, longest_sequence.first] # => [3, 4]
The solution below is likely most efficient since it is O(N). It walks through an array, collecting the chunks:
arr.each.with_index.reduce({idx:-1, i: -1, len: 0}) do |memo, (e, i)|
memo[:i] = i if memo[:i] == -1 && e == 2 # at the beginning of chunk
memo[:len], memo[:idx] = [i - memo[:i], memo[:i]] \
if memo[:i] >= 0 && i - memo[:i] > memo[:len] # save values if needed
memo[:i] = -1 unless e == 2 # reset index counter
memo
end.reject { |k, _| k == :i } # reject temporary index value
#⇒ {
# :idx => 4,
# :len => 3
# }
To use it as method, accepting a parameter; just wrap the code above with def find_duplicates number and substitute 2 with number in the code above. Yes, it returns hash instead of an array.

A method for an array

What is the most concise and explicit way to write a method for this?
Given an array a of numbers and a number n, find the n consecutive elements of a whose sum is the largest.
Return the largest sum and the index of the first element in the group.
For example, with a = [1, 1, 1, 1, 1, 1, 1, 2] and n = 2, the
result would be a sum 3 and position 6.
arr = [1,3,2,4,3,5,2,1,3,4,2,5,1]
size = 3
Inefficient but pretty
arr.each_cons(size).with_index.map { |a,i| [a.inject(:+), i] }.max_by(&:first)
#=> [12, 3]
Efficient but whupped with an ugly stick1
tot = arr[0,size].inject(:+)
(1..arr.size-size).each_with_object([tot, 0]) do |i, best|
tot += arr[i+size-1] - arr[i-1]
best.replace([tot, i]) if tot > best.first
end
#=> [12, 3]
Steps performed by the pretty one
enum0 = arr.each_cons(size)
#=> #<Enumerator: [1, 3, 2, 4, 3, 5, 2, 1, 3, 4, 2, 5, 1]:each_cons(3)>
enum1 = enum0.with_index
#=> #<Enumerator: #<Enumerator: [1, 3, 2, 4, 3, 5, 2, 1, 3, 4, 2, 5, 1]:
# each_cons(3)>:with_index>
Carefully examine the above return value for enum1. You will see it is effectively a "compound" enumerator. We can see the values that enum1 will generate and pass to map by converting it to an array:
enum1.to_a
#=> [[[1, 3, 2], 0], [[3, 2, 4], 1], [[2, 4, 3], 2], [[4, 3, 5], 3],
# [[3, 5, 2], 4], [[5, 2, 1], 5], [[2, 1, 3], 6], [[1, 3, 4], 7],
# [[3, 4, 2], 8], [[4, 2, 5], 9], [[2, 5, 1], 10]]
Continuing:
b = enum1.map { |a,i| [a.inject(:+), i] }
#=> [[6, 0], [9, 1], [9, 2], [12, 3], [10, 4], [8, 5],
# [6, 6], [8, 7], [9, 8], [11, 9], [8, 10]]
Note the since the first element of enum1 that map passes to the block is [[1, 3, 2], 0], the two block variables are assigned as follows (using parallel or multiple assignment):
a, i = [[1, 3, 2], 0]
#=> [[1, 3, 2], 0]
a #=> [1, 3, 2]
i #=> 0
and the block calculation is performed:
[a.inject(:+), i]
#=> [6, 0]
Lastly,
b.max_by(&:first)
#=> [12, 3]
Enumerable#max_by determines the largest value among
b.map(&:first)
#=> [6, 9, 9, 12, 10, 8, 6, 8, 9, 11, 8]
Steps performed by the less pretty one
a = arr[0,size]
#=> [1, 3, 2]
tot = a.inject(:+)
#=> 6
enum = (1..arr.size-size).each_with_object([tot, 0])
#=> (1..13-3).each_with_object([6, 0])
#=> #<Enumerator: 1..10:each_with_object([6, 0])>
enum.to_a
#=> [[1, [6, 0]], [2, [6, 0]], [3, [6, 0]], [4, [6, 0]], [5, [6, 0]],
# [6, [6, 0]], [7, [6, 0]], [8, [6, 0]], [9, [6, 0]], [10, [6, 0]]]
enum.each do |i, best|
tot += arr[i+size-1] - arr[i-1]
best.replace([tot, i]) if tot > best.first
end
#=> [12, 3]
The first element of enum, [1, [6, 0]], is passed to the block, assigned to the block variables and the block calculation is performed:
i, best = [1, [6, 0]]
#=> [1, [6, 0]]
i #=> 1
best
#=> [6, 0]
tot += arr[i+size-1] - arr[i-1]
# tot = 6 + arr[1+3-1] - arr[1-1]
# = 6 + 4 - 1
# = 9
best.replace([tot, i]) if tot > best.first
#=> best.replace([9, 1]) if 9 > 6
#=> [9, 1]
best
#=> [9, 1]
The remaining calculations are similar.
1 Credit to Bo Diddley (at 2:51)

Ruby grouping elements

I have array:
a = [1, 3, 1, 3, 2, 1, 2]
And I want to group by values, but save it indexes, so result must be looks like this:
[[0, 2, 5], [1, 3], [4, 6]]
or hash
{1=>[0, 2, 5], 3=>[1, 3], 2=>[4, 6]}
Now I'm using pretty ugly and big code:
struc = Struct.new(:index, :value)
array = array.map.with_index{ |v, i| struc.new(i, v) }.group_by {|s| s[1]}.map { |h| h[1].map { |e| e[0]}}
`
If you use a hash with a default value to avoid iterating twice over the elements:
a = [1, 3, 1, 3, 2, 1, 2]
Hash.new { |h, k| h[k] = [] }.tap do |result|
a.each_with_index { |i, n| result[i] << n }
end
#=> { 1 => [0, 2, 5], 3 => [1, 3], 2 => [4, 6] }
a = [1, 3, 1, 3, 2, 1, 2]
a.each_with_index.group_by(&:first).values.map { |h| h.map &:last }
First we get an Enumerator in the form [val, idx], ... (each_with_index), then group_by the value (first value in pair), then take the index (last element) of each pair.
You can use:
Enumerable#each_with_index
Enumerable#group_by and
Array#transpose:
a = [1, 3, 1, 3, 2, 1, 2]
a.each_with_index.group_by(&:first).values.map { |b| b.transpose.last }
#=> [[0, 2, 5], [1, 3], [4, 6]]

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